Power generating machines such as gas and steam turbine engines are widely used throughout the world, both on land and at sea. These machines include those which provide electricity for residential as well as industrial uses, and for providing propulsion to naval vessels and aircraft.
Certain components used in the high temperature section of power generating turbines undergo hot corrosion and oxidation during service. Hot corrosion generally takes place at temperatures in the range of about 700.degree.-930.degree. C. (1,300.degree.-1,700.degree. F.), when certain compounds which deposit on the component surfaces melt. These compounds contain elements such as the alkalis and alkaline earths, vanadium, lead, sulfur and oxygen, which are present in the air and fuel consumed by the engine during operation. Oxidation takes place at temperatures both above and below those at which hot corrosion takes place, and in the absence of compounds of the type mentioned above. Oxidation degradation is normally less rapid than degradation produced by hot corrosion.
Hot corrosion and oxidation requires the operators of these turbines to expend large amounts of time and money in repairing or replacing damaged components. As a result, the industry has searched for materials which can provide resistance to such attack. One solution which has been used to date has been to apply coatings to the surface of the engine components.
Coatings which form oxide scales based on chromium, typically Cr.sub.2 O.sub.3, are particularly effective in reducing the amount of hot corrosion which takes place during engine operation. They also provide protection against oxidation. Chromium-based oxide scales, while reducing the amount of corrosion and oxidation on turbine components, sometimes spall or exfoliate from the surface of the components. After a sufficient amount of spallation, the chromium oxide film can no longer reform because the activity of chromium is reduced below the required level. When chromium levels are depleted, less protective and faster forming oxides form. Sometimes the term "green rot" is used to describe this condition. Further, vaporization of the chromium oxide at elevated temperatures limits the use of Ni-Cr coatings.
In view of the need for chromium oxide forming coatings with better properties, researchers have sought to improve the scale adherence of chromia films and to increase the temperatures to which they may be used. This invention satisfies those needs.